The fungus associated with S. noctilio F. in Australia is identified as Amyloster-eum areolatum (Fr.) Boidin. By using a polystyrene film to prevent the breakup of the chains of arthrospores during staining, it was possible to demonstrate that the spores are homokaryotic though usually multinucleate. Viable homokaryons were established from arthrospores by using a tissue culture medium incorporating growing callus tissue of Pinus radiata. Homokaryons and heterokaryons of the S. noctilio fungus were opposed in all possible combinations against homokaryons and heterokaryons of A. chailleti'i (Pers. ex Fr.) Boidin, A.laevigatum (Fr.) Boidin, and A. areolatum, the only known species of Amylostereum. The S. noctilio fungus was interfertile only with A. areolatum, as judged by production of anastomoses, clamped mycelium, and fertile fruit bodies. Starch-gel electrophoresis showed that the soluble proteins of the S. noctilio fungus corresponded more closely with those of A. areolatum than with those of A. chailletii.